1. Field of the Invention
The invention relates to an EGR cooler system for an exhaust gas recirculation system. The exhaust gas recirculation system sends part of the exhaust gas from an exhaust passage of an internal combustion engine back to an intake passage of the internal combustion engine. The EGR cooler system cools the exhaust gas to be sent back to the intake passage using a water-cooled EGR cooler.
2. Description of the Related Art
Usually, generation of NOx is suppressed as follows; part of the exhaust gas is extracted and cooled by a water-cooled EGR (Exhaust Gas Recirculation) cooler, and the cooled exhaust gas is sent back to an intake passage of the internal combustion engine to reduce the combustion temperature.
In such exhaust gas recirculation system, the EGR cooler is supplied with the coolant from the engine cooling system that cools a heated portion of the internal combustion engine. However, when a vehicle is stuck in heavy traffic, the coolant, which has been used to cool the heated portion of the internal combustion engine, is sometimes not sufficiently cooled and the temperature of the coolant remains high (for example, approximately 80° C.). If such high-temperature coolant is supplied to the EGR cooler, the exhaust gas cannot be sufficiently cooled.
According to Japanese Patent Application Publication No. JP-A-2004-204828, the coolant to be supplied to the EGR cooler is cooled by an EGR cooler system provided in a path that is separate from the path provided with the engine cooling system. The engine cooling system cools the coolant to be supplied to a heated portion of an internal combustion engine.
According to this technology, however, the coolant is sometimes excessively cooled by the EGR cooler system, for example, when the vehicle is running at a high peed or when the vehicle is in a cold environment. If such excessively cooled coolant is supplied to the EGR cooler, the exhaust gas is also excessively cooled, resulting in an excessive decrease in the temperature of an intake air in an intake passage. This may cause misfires in the internal combustion engine, adversely affecting the drivability.